Sensors, such as glucose sensors, for detecting and measuring desired characteristics, such as glucose content, of liquid samples are well-known. Whenever these sensors need to be calibrated, the liquid sample which normally contacts the sensor is replaced with one or more reference liquids or calibration samples. While this is usually a simple procedure, it can present complications when the analysis is being conducted on blood from human or animal subjects. In a typical situation for continuous blood sampling, a double lumen catheter is inserted into a cannula positioned in a vein or artery of the subject. The double lumen catheter is connected through appropriate pumping apparatus to a sensor. In the prior art, whenever the sensor needed calibration, the double lumen catheter would be removed from the cannula and inserted into a container having a desired reference liquid therein. After the calibration is finished, the double lumen catheter is then replaced in the cannula for further analytical use. This prior art procedure can have the disadvantage of losing the desired sterility conditions for the catheter and cannula. It also has the disadvantage that the cannula may become clogged with coagulated blood while the catheter is removed. The cannula would then have to be flushed or replaced with attendant discomforture to the patient.
There is thus a need for apparatus for calibration of a sensor during use which does not interfere with the usual sampling procedure.